Surface Fe2 Research Articles

Surface structural controls on pyrite oxidation kinetics; an XPS-UPS, STM, and modeling study

X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS, respectively) and scanning tunneling microscopy (STM) were used to observe the initial oxidation of pyrite surfaces in air. The results show the growth of oxide-like oxidation products, with minor contributions from sulfate. UPS shows a decrease in the density of electronic states in the uppermost valence band of pyrite, corresponding to oxidation of surface Fe2+. This allows reliable interpretation of STM images, which show that initial surface oxidation of FeH proceeds by growth of oxidized patches. The borders of oxidized patches contain small segments oriented in the (110) and (100) directions. STM of as-received pyrite cube surfaces, oxidized in air for years, also show the importance of the (110) crystallographic directions, on the surface, in controlling reaction progress. A model in which oxidation probabilities for FeH surface sites are proportional to the number of nearest-neighbor oxidized (FeH) sites was tested using a Monte Carlo approach and reproduces the surface patterns observed in STM. An oxidation mechanism consistent with the XPS, UPS, STM, and Monte Carlo results is proposed. The rate constant for electron transfer from surface-exposed pyrite FeH to O2 is small. Electron transfer is more rapid from pyrite FeH to FeH present on the surface as an oxidation product, such as in the patches we observed. FeH in oxide is a better reductant than FeH in pyrite, so electron transfer to O2 from the oxide is also fast. However, this two-step mechanism is faster overall only if electron transfer to the surface oxide patches is irreversible (e.g., because ofS2 oxidation or electron hopping within the surface oxide patches). Cycling of Fe between the FeH and FeH forms, particularly along borders between oxidized and unoxidized areas, is thus a key feature of the pyrite oxidation mechanism. An understanding of the surface electronic and band structure aids definition of the redox potentials of electrons in various surface states. Rates of electron transfer from these states to O2 are estimated using a kinetic theory of elementary heterogeneous electron transfer. 1994), reduction of aqueous trace metal complexes to form ore deposits (Jean and Bancroft 1985; Bakken et al. 1989), and nutrient and metal cycling at oxic-anoxic boundaries on lake bottoms and in estuaries (Morse 1994). Pyrite oxidation is also important in technological applications ranging from hydrometallurgy (Buckley and Woods 1987; Karthe et al. 1993) to solar energy conversion (Ennaoui et al. 1986). Geochemical understanding of pyrite oxidation is largely based on wet-chemical studies of overall rates and stoichiometries. Important oxidants (e.g., O2 and FeH) have been identified and widely applicable rate laws developed (e.g., Singer and Stumm 1970; Williamson and Rimstidt 1994). Reaction mechanisms are less well * Present address: Department of Geology and Geophysics, known. Predicting and controlling the environmental asUniversity of Wyoming, Laramie, Wyoming 82071-3006, U.S.A. pects of pyrite oxidation, such as the role ofFeH at pyrite 0003-004X/96/091 0-1 036$05.0

Direct conversion of methane and ethane to the corresponding alcohols using nitrous oxide over iron phosphate catalysts

The partial oxidation of methane and ethane by nitrous oxide over iron phosphate catalysts has been studied at 573–723 K. Methanol and ethanol were obtained as the primary products from methane and ethane, respectively. Further oxidation of methanol leads to the formation of formaldehyde, CO and CO2 as the consecutive oxidation products. In the case of ethane oxidation, the primary product, ethanol, undergoes dehydration and oxidation to ethene and acetaldehyde, respectively.An excess of either iron or phosphorus in the FePO4 catalysts was disadvantageous to their catalytic activities. It was suggested that an iron site together with the surrounding phosphates was important for the reactions. The most appropriate Fe/P ratio on the surface was 0.4–0.5. X-Ray photoelectron spectroscopy (XPS) studies suggested that the active site was Fe2+ on the FePO4 surface. The pulse experiments showed that the pre-reduction of the catalyst surface by either hydrogen or carbon monoxide greatly increased the conversion of methane and the yield of methanol. Co-feeding of hydrogen in a reactant gas flow also increased the yields of methanol and of ethanol for the oxidations of methane and ethane, respectively. In the case of methane oxidation, the yield of methanol was decreased markedly with the addition of water. In the case of ethane oxidation, the yield of ethanol increased at a low partial pressure of water, owing to the suppression of the dehydration of ethanol to ethene.It is suggested that an active oxygen species (O*) formed on the surface Fe2+ site is responsible for the oxidation of both methane and ethane. Methane or ethane is activated by the O* in cooperation with the neighbouring phosphates, forming a methoxide or ethoxide as an intermediate. Methanol or ethanol is then produced through the reaction of the intermediate with a highly acidic proton on the neighbouring phosphate unit.

Interaction of O 2 with the Fe 0.84Cr 0.16(001) surface studied by photoelectron spectroscopy

Abstract Soft X-ray photoelectron spectroscopy (SXPS; hv = 130−600 eV) employing synchrotron radiation and X-ray photoelectron spectroscopy (XPS; hv = 1486.6 eV) employing AlKα X-rays were used to study the effect of molecular oxygen adsorption (1 to 104 L) on the (001) surface of the steel-like alloy Fe0.84Cr0.16 at room temperature. The use of multiple photon energies allowed a qualitative determination of the distribution of species in the oxide film perpendicular to the surface. Prior to oxidation, Fe0.84Cr0.16samples were annealed to different temperatures to produce varying concentrations of Cr in the top few monolayers of the (001) surface. The differences between the oxide films produced on these different surfaces were determined from SXPS primarily of the Fe 3p, Cr 3p, and valence levels. Small amounts of carbidic carbon(⩽ 1 ML) that had segregated to the (001) surface of the Fe0.84Cr0.16alloy during initial sputter/anneal treatments were removed during the first ∼ 5 L O2 exposure. In general, the resistance of the Fe0.84Cr0.16(001) surface to oxidation was directly related to the initial Cr concentration at the surface. The oxide films were richer in Cr compared to the bulk concentration for the lowest O2 exposures, and became increasingly enriched in Fe for increasing exposures, in agreement with studies by other groups. The resulting mixed FeCrO film contained Cr that was in the form of Cr3+ for all exposures. The Fe was in the form of Fe2+ and Fe3+, with an increasing Fe3+/Fe2+ ratio as the O2 exposure is increased. The results indicate that for O2 exposures

Thermally Oxidized Iron Electrodes for Photoelectrochemical Application

AbstractAir‐oxidized and CO2‐treated laminar iron electrodes have been characterized structurally (X‐ray diffraction) and photoelectrochemically (photocurrent, polarization, cyclic voltammetry, flat‐band and static potential, band‐gap and cell characteristics) in order to elucidate the role of Fe2+ and Fe3+ at the semiconductor‐electrolyte interface in the photooxidation of H2O. Surface Fe2+ increases the band‐gap and the electron‐hole recombination rate; Fe3+ causes electroreduction of O2, which is followed by oxidation of Fe2+ (Fe3O4 → Fe2O3). This affects the catalytic activity for dissociative water adsorption as Fe2+ on non‐stoichiometric α‐Fe2O3 acts as the active site.

The reduction of hydrogen peroxide on passive iron in alkaline solutions

The electrochemical reduction and catalytic decomposition rates of hydrogen peroxide on passive iron in alkaline solutions have been studied. The reaction orders with respect to H2O2 and OH− ions found were 1 and — 1 respectively, but the latter value reflects the equilibrium properties of the active site on the oxide layer. At the r.d.s., the OH− ion reaction order is zero. No difference in the behaviour of HO2− or H2O2 was found and a common reduction pathway is proposed. The reduction mechanisms proposed involve either the coordination of the peroxide ion on a Fe3+ centre, followed by reduction on the adsorbed state, or an ECE sequence where the reduction is mediated by surface Fe2+ centres. The reduction of alkaline solutions of periodate on passive iron and platinum shows striking similarities with solutions of peroxide. The proposed mechanisms are also probably valid for anions that can be either coordinated to Fe3+ centres or else can react with surface Fe2+, but which are reduced through a chemical step on a noble metal. The rate of catalytic decomposition of H2O2 on passive iron is very low, less then 8×10−11 mol cm−2 s−1, a result which was found to be in agreement with the very low value of the reduction rate constant found, kθ=1×10−15 cm−2 s−1 at unit activity of OH− and H2O2 and at the standard potential of the H2O2/OH− couple.